1. Technical Field
The present invention relates to a method of operating a power generation system using a fuel cell including molten carbonate, and particularly relates to an operation method for such a system to achieve a high efficiency power generation.
2. Background Art
A known molten carbonate-type fuel cell generally includes a plurality of fuel cell elements. Each fuel cell element includes an electrolyte plate (tile), a cathode (oxide electrode) and an anode (fuel electrode). The electrolyte plate includes a porous substance soaked with molten carbonate, and the cathode electrode and the anode electrode sandwich the electrolyte plate. A cathode chamber is formed in the cathode electrode and an anode chamber is formed in the anode electrode. Oxidizing gas is fed to the cathode chamber whereas fuel gas is fed to the anode chamber to cause power generation due to an electric potential difference between the cathode electrode and the anode electrode. A plurality of fuel cell elements are stacked one after another with separators being interposed between each two fuel cell elements.
Among power generation systems using the fuel cell of the above-mentioned type, a typical natural gas reforming type power generation system is illustrated in FIG. 14. An electrolyte plate 1 is sandwiched by a cathode and an anode. A cathode chamber 2 is provided over the cathode and an anode chamber 3 is provided over the anode to form a fuel cell I. Part of air A is pressurized by a compressor 4, cooled by a cooling device 5, pressurized again by another compressor 6 and preheated by an air preheater 7 before introduced to the cathode chamber 2 through an air feed line 8. Remaining part of the air A is introduced to a combustion chamber of a reformer 10 by a branch line 9. Gases discharged from the cathode chamber are introduced to a turbine 12 through a cathode exhaust gas line 11 and then expelled to atmosphere via the air preheater 7.
On the other hand, natural gas (CH.sub.4) NG flows through a natural gas preheater 13 and 14, a desulfurizer 15 and the reformer 10. The natural gas NG is reformed to fuel gas FG in the reformer 10 and the fuel gas GF is fed to the anode chamber 3 of the fuel cell I. Gases discharged from the anode chamber 3 are led to a heat exchanger 17, the natural gas preheater 14, a heating device 18, the natural gas preheater 13 and a condensor 19. The gases are cooled in the condensor 19 and introduced to a gas-liquid separator 20. In the gas-liquid separator 20, H.sub.2 O is separated from the gases (anode exhaust gas), and then the gases are pressurized by a blower 21, introduced to the combustion chamber of the reformer 10 through a line 22 extending to the heat exchanger 17 and fed into the anode chamber 2 from the reformer 10. The separated H.sub.2 O is compressed by a pump 23 and led to a water heater 24 to become steam. The steam flows through the steam generator 18 and merges with the natural gas NG at an entrance of the reformer 10. Numeral 26 designates a blower used for a cathode recirculation.
When the molten carbonate fuel cell is operated for power generation, the natural gas (CH.sub.4) is reformed and fed to the anode chamber 3. In this case, a following reaction takes place in the reformer 10: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2
In the cathode chamber 2 of the fuel cell I, on the other hand, a following reaction takes place: EQU CO.sub.2 +1/2O.sub.2 +2e.sup.- .fwdarw.CO.sub.3.sup.--
In the latter reaction, a carbonate ion CO.sub.3.sup.-- is produced and this carbonate ion CO.sub.3.sup.-- reaches the anode via the electrolyete plate 1. In the anode, the fuel gas reformed by the reformer 10 is introduced and contact the carbonate ion CO.sub.3.sup.--, thereby causing following reactions: EQU CO.sub.3.sup.-- +H.sub.2 .fwdarw.CO.sub.2.sup.-- +H.sub.2 O+2e.sup.- EQU CO.sub.3.sup.-- +CO.fwdarw.2CO.sub.2 +2e.sup.-
Therefore, 5CO.sub.2 and 3H.sub.2 O are discharged as the anode exhaust gas. In this manner, upon the power generation in the molten carbonate fuel cell, CO.sub.2 is absorbed in the cathode whereas the same amount of CO.sub.2 is produced in the anode. This can be said, at least apparently, that CO.sub.2 is separated from the cathode gas and the separated CO.sub.2 is transferred to the anode.
In the reaction at the cathode 2, if the CO.sub.2 concentration at the cathode is high, the reaction is accelerated and the fuel cell voltage becomes higher so that the power generation efficiency will be raised.
However, in the conventional molten carbonate type fuel cell system, CO.sub.2 among the anode exhaust gas discharged from the anode chamber 3 is introduced to the cathode chamber 2 to feed necessary CO.sub.2 to the cathode chamber 2 and CO.sub.2 recirculatedly fed to the cathode chamber 2 is diluted by the air. Therefore, the CO.sub.2 concentration at the entrance of the cathode chamber 2 is very low, for example 7% which is considerably lower than a standard concentration of 30%. Consequently, the cell voltage is lower.